Field of the Invention
The present invention relates to a technique to adjust a write start position of laser beam in an electrophotographic image forming apparatus.
Description of the Related Art
An electrophotographic image forming apparatus has an optical scanning unit for exposing a photoconductor. The optical scanning unit irradiates and exposes a photoconductor with laser beam by emitting the laser beam based on image data, causing a rotating polygon mirror to reflect the emitted laser beam, and causing the laser beam to pass through a scanning lens. Then, a latent image is formed on the photoconductor by performing scanning to move the spot of the laser beam formed on the surface of the photoconductor by rotating the rotating polygon mirror.
Normally, as a scanning lens, a lens having so-called fθ characteristics is used. Here, the fθ characteristics are optical characteristics that form an image of laser beam on the surface of a photoconductor by moving the spot of the laser beam on the surface of the photoconductor at a uniform speed on the surface of the photoconductor while a rotating polygon mirror is rotating at a uniform angular speed. By using a scanning lens having such fθ characteristics, it is possible to perform appropriate exposure. However, the scanning lens having the fθ characteristics is comparatively large and expensive. Because of this, with a view to downsizing the image forming apparatus and to reducing its cost, not using the scanning lens itself or using a scanning lens not having the fθ characteristics has been discussed.
In the case where a scanning lens not having the fθ characteristics is used, the spot of laser beam does not move on the surface of a photoconductor at a uniform speed in the scanning direction (main scanning direction) in which the laser beam scans on the surface of the photoconductor. Because of this, there is such a problem that the width of a dot formed on the photoconductor surface is not uniform. For this problem, there has been proposed a technique to form a latent image equivalent to that in the case where the spot of laser beam scans on the surface of a photoconductor at a uniform speed by correcting the image data corresponding to each exposure position of the laser beam in the main scanning direction in accordance with the exposure position. For example, Japanese Patent Laid-Open No. 2005-096351 has disclosed a technique to add image data to image data configuring a PWM lighting pattern or to remove image data from the PWM lighting pattern so that an image with a desired width is formed on the surface of a photoconductor in an electrophotographic image forming apparatus. The image forming apparatus described in Japanese Patent Laid-Open No. 2005-096351 generates 16-bit bit data from density data representing density tone levels. The image forming apparatus adds bit data to the generated 16-bit bit data or deletes bit data from the 16-bit bit data.
As described above, in an image forming apparatus using a scanning lens not having the fθ characteristics, laser beam on the surface of a photoconductor does not move at a uniform speed. Because of this, the scanning movement amount based on an image clock that serves as a reference of scanning varies depending on the scanning position. That is, the distance traveled by laser beam in one period of the image clock varies depending on the scanning position. The adjustment of the scanning position is performed by taking the image clock as a reference, and therefore, the unit of the adjustment amount also varies depending on the scanning position.
Regarding this point, the technique disclosed in Japanese Patent Laid-Open No. 2005-096351 described above does not take into consideration the change in the unit of the adjustment amount at the time of scanning position adjustment. Consequently, in the case where an attempt is made to start scanning of laser beam from a specified position, the scanning start position deviates due to the change in the unit of the adjustment amount. Then, in a general image forming apparatus, an arbitrary scanning start position is specified so that the center position of a latent image to be formed on the photoconductor and the center position of a printing medium align with each other, and therefore, the above-described deviation in the scanning start position will result in a deviation in the printing position of an image to be formed on the printing medium.
Further, even in the case where the same scanning start position is specified, there may be a slight variation in the scanning speed of laser beam on the photoconductor surface among image forming apparatuses due to the factor, such as an assembling error of each part. In this case, it is necessary to set an adjustment amount in accordance with the scanning speed characteristics in each individual image forming apparatus, but Japanese Patent Laid-Open No. 2005-096351 described above does not take into consideration the individual difference such as this among the image forming apparatuses.